Proportional motor actuator circuit

ABSTRACT

A motor actuator circuit for use in heating and air conditioning and process control systems to selectively rotate a motor shaft which positions a damper or valve. A command signal, such as a voltage, or a potentiometer controlled by a sensing element of the system provides a signal representing the desired position for the damper and a sense potentiometer provides a signal representing the actual shaft position; a signal comparator circuit including a differential amplifier having an output switching stage is responsive to a potential difference between the signals to effect energization of one of a pair of motor windings, the energized one of the pair being determined by the polarity of the difference signal; and detent circuits derive a detent signal from the voltage across the other one of the motor windings for coupling to the amplifier input to increase the potential difference, the sense potentiometer wiper being linked to the motor shaft so that the potential difference decreases as the shaft rotates to the desired position, and the motor is deenergized when the desired position is reached.

United States Patent 1 Nettles June 26, 1973 PROPORTIONAL MOTOR ACTUATORCIRCUIT [75] lnventor: Robert G. Nettles, Goshen, Ind. [73] Assignee:Johnson Service Company,

Milwaukee, Wis.

[22] Filed: Jan. 15, 1971 [21] Appl. No.: 106,650

[52] US. Cl. 318/678, 318/615, 318/683 [51] Int. Cl. G05f 1/00 [58]Field of Search 318/678, 683, 615,

[56] References Cited UNITED STATES PATENTS 3,652,913 3/1972 Leland318/678 3,571,688 3/1971 Tomasulo et al.... 318/681 X 3,391,317 7/1968Bell 318/611 2,821,674 l/l958 Hughes 318/207 2,886,642 5/1959 Ehret etal.... 318/616 3,510,739 5/1970 Peterson 318/663 X Primary Examiner--B.Dobeck Attorney-Johnson, Dienner, Enrich, Verbeck &

Wagner [5 7] ABSTRACT A motor actuator circuit for use in heating andair conditioning and process control systems to selectively rotate amotor shaft which positions a damper or valve. A command signal, such asa voltage, or a potentiometer controlled by a sensing element of thesystem provides a signal representing the desired position for thedamper and a sense potentiometer provides a signal representing theactual shaft position; a signal comparator circuit including adifferential amplifier having an output switching stage is responsive toa potential difference between the signals to effect energization of oneof a pair of motor windings, the energized one of the pair beingdetermined by the polarity of the difference signal; and detent circuitsderive a detent signal from the voltage across the other one of themotor windings for coupling to the amplifier input to increase thepotential difference, the sense potentiometer wiper being linked to themotor shaft so that the potential difference decreases as the shaftrotates to the desired position, and the motor is deenergized when thedesired position is reached.

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PROPORTIONAL MOTOR ACTUATOR CIRCUIT BACKGROUND OF THE INVENTION 1. Fieldof the Invention This invention relates to analog control circuits, andmore particularly, to proportional motor actuator circuits for use inheating, air conditioning and process control systems.

2. Description of the Prior Art In analog control systems, a monitoringdevice is used to continuously monitor a physical characteristic of asystem and provide an electrical analog signal representing the currentvalue of the characteristic. The signal provided is compared to a signalrepresenting a desired value or set point for the characteristic, andwhen the compared signals differ by a predetermined amount, an errorsignal, which is the difference between the signals being compared,controls system apparatus to compensate for the error.

For example, in heating and air conditioning systems, analog signals maybe derived through the use of a monitoring device comprising abimetallic element, a diaphragm, bellows, etc., which continuouslymonitor the characteristic of the system, such as temperature or steampressure, and provide analog signals as inputs to a motor actuatorcircuit for controlling operation of a motor.

The motor shaft is connected to drive a damper located in an air duct,or a valve connected in a hot water or steam line, such that controlledoperation of the motor effects changes in the system characteristicbeing monitored thereby compensating for the error detected.

To provide such compensation, the motor shaft must be rotatable in aclockwise or a counterclockwise direction within a predetermined range,and the error signal must indicate the amount and direction of rotationneeded to move the shaft and correspondingly a damper or valve to adesired setting. Accordingly, some prior art motor actuator circuitshave used electromechanical balance relays which sense both theamplitude and phase or polarity of the analog signal provided by themonitoring device and control the energization of the motor winding viarelay contacts. However, switching thereby the large inductive currentsrequired to energize the motor winding causes undesirable arcing andburning at the contacts. Moreover, control circuits employing relays aresubject to line fluctuations and power dissipation in the relay coil.

Solid state switching circuits provided for motor control purposes toavoid disadvantages caused by mechanical contacts and power dissipationare more expensive than electrical mechanical relays they replace.Moreover, these circuits are generally more susceptible to line voltagevariations and have the further disadvantage of requiring a large heatsink for the solid state switching elements used in the circuits.

SUMMARY OF THE INVENTION The present invention provides a motor controlcircuit including solid state switching elements for selectivelyenergizing windings of a two phase motor to rotate the motor shaft to adesired position in accordance with a command provided by a monitoringdevice which indicates the desired position for the shaft.

The control circuit comprises an error sensing network including commandmeans responsive to the command to provide a command signal representingthe desired position of the shaft, position sense means providing asense signal representing the actual position of the shaft, andcomparator means for comparing the sense signal and the command signalto determine the direction and amount of rotation required to move themotor shaft to the desired position. When the shaft is in the desiredposition, the sense signal and the command signal provided to thecomparator means are equal and the motor windings are not energized. Onthe other hand, when the shaft is not at the desired position, the senseand command signals are not equal and the comparator means accordinglyprovides a control signal for enabling drive circuits, controlled by thecomparator means, to effect clockwise or counterclockwise rotation ofthe motor shaft to the desired position.

The position sense means are controlled by the motor shaft to provide asignal that is proportional to the shaft position such that once themotor is energized, the shaft will be rotated until the sense signal isequal to the command signal at which time the motor will be deenergized.

In accordance with one feature of the invention, when one of the motorwindings is energized, a detent signal derived from the potential acrossthe other motor winding is provided to the comparator means to enhanceinequality between the command and sense signals. Accordingly, once adrive circuit is enabled by the comparator means to energize one of themotor windings, the detent signal causes the enabled drive circuit to belocked on" until the sense signal equalizes both the change in thecommand signal and the detent signal thereby assuring compensation forthe error indicated by the change in the command signal.

In one embodiment for a motor control circuit provided by the presentinvention, the command controller means and the sense controller meanscomprise first and second potentiometers connected in parallel acrossoutputs of a D.C. voltage source forming a bridge circuit, the output ofwhich is taken from the wipers of the command potentiometer and sensepotentiometers. The D.C. potential at the wiper of the commandpotentiometer represents the desired position for the shaft and the D.C.potential at the wiper of the sense potentiometer represents the actualposition of the shaft.

A novel comparator means comprising a D.C. differential amplifier has afirst input connected to the wiper of the command potentiometer and asecond input connected to the wiper of the sense potentiometer. In abalanced condition, that is when the shaft is in the desired position,the signals input to the differential amplifier are equal and in anunbalanced condition, when the shaft is not in the desired position, oneof the inputs to the differential amplifier will be greater than theother input. An inherent feature of the differential amplifiercomparator means is that one of the input signals must exceed the otherin a predetermined amount before the motor drive will be energized.

The differential amplifier further comprises an output stage includingfirst and second output switching transistors inversely connected to theinput stage transistors such that when one of the input signals to theamplifier is greater than the other input signal, one output transistoris forward biased and the other output transistor is reversed biased. Adrive circuit associated with the forward biased transistor will beenabled to effeet energization of one of the windings of the motor andthe drive circuit associated with the reverse biased transistor will bedisabled.

Thus, when the amplitude of the command signal is greater than theamplitude of the sense signal, one drive circuit will be enabled toeffect energization of one of the windings, and when the amplitude ofthe command signal is less than the amplitude of the sense signal, theother drive circuit will be enabled to effect energization of the othermotor winding.

The wiper of the command potentiometer is mechanically coupled tomonitoring means which positions the wiper as a function of a monitoredvariable.

The wiper of the sense potentiometer is mechanically coupled to themotor shaft and is moved by the motor shaft as a function of theposition of the shaft. Thus, movement of the command controller wipercauses energization of one of the motor windings effecting movement ofthe wiper of the sense potentiometer, the motor winding being energizeduntil the wiper of the sense potentiometer has been moved sufficientlyto overcome the change in potential at the controller potentiometerresulting from the command monitoring device.

Detent feed back means are connected between one of the windings and afirst input of the differential amplifier, and further detent feed backmeans are connected between the other winding of the motor and thesecond input of the differential amplifier to provide the desired detentsignal whenever one of the motor windings is energized.

A further feature of the novel motor actuator circuit is that means areprovided to assure that the sense potentiometer can provide a sensesignal to the differential amplifier which will compensate for unbalanceconditions caused by movement of the controller potentiometer. Thus,mechanical limit switches are not necessary to prevent the controllerpotentiometer from causing the feed back potentiometer to be driven toits resistance limits. I

In accordance with a further feature of the invention, the motor controlcircuit includes adjustable travel limit means for the controllerpermitting adjustment of the amplitude of the sense signal such that thewiper of thesense controller potentiometer, which is driven directly bythe motor shaft, is required to travel, for example, only one-third ofits range to provide the sense signal required to balance a change inthe command signal.

Another feature of the invention is that over-signal clamping means areprovided to assure that the sense controller potentiometer will have aslightly greater signal than the input signal provided by the commandcontroller potentiometer so that a balance can be obtained in the eventofa short circuit or an overload condition occurring in the motoractuator circuit.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a block diagram ofa motoractuator circuit provided by the present invention;

FIGS. 2 and 3 which when assembled in side by side relationshipillustrate a schematic circuit diagram of the motor actuator circuitshown in block form in FIG. I; and

FIG. 4 is a schematic circuit diagram of a circuit for connection intothe actuator circuit shown in FIGS.

and 3 providing an alternative embodiment for the differential amplifierof the actuator circuit.

DESCRIPTION OF PREFERRED EMBODIMENTS General Description A block diagramof a motor actuator circuit 20 provided by the present invention isshown in FIG' 1. As there shown, the actuator circuit 20 is used toeffect selective energization of windings 21 and 22 of a motor 23 torotate the shaft or rotor 24 of the motor to a desired position.

For example, in a temperature conditioning system the motor may providecontrolled positioning of a damper 25, driven by the motor shaft 24, asa function of a command provided to the motor actuator circuit 20 by wayof an analog sensing device 26 which monitors the temperature at onelocation within the temperature conditioning system.

The motor actuator circuit employs D.C. sensing to compare signalsprovided by command controller means 30 and sense controller means 40 toeffect positioning of the motor shaft.

In FIG. 1, command controller means 30 controlled by analog sensingmeans 26 include a potentiometer R22 connected between the outputterminals 32 and 33 of a regulated D.C. voltage source 34 which derivesa D.C. voltage from a 60Hz, volt A.C. power line coupled throughtransformer T1. The wiper 36 of potentiometer R22 is mechanicallyconnected to the sensing device 26 and movable thereby such that theD.C. potential at wiper 36 of the command potentiometer R22 representsthe desired position of the motor shaft 24 which position is a functionof the temperature monitored by the analog sensing device 26.

Shaft position sensing controller means 40 includes sense potentiometerRI connected between the output terminals 32 and 33 of the voltagesource 34 in parallel with the command controller means 30. The wiper 4]of feedback potentiometer Rll is mechanically connected to the motorshaft 24 and movable thereby such that the D.C. voltage at wiper 41represents the actual position of the motor shaft 24 (and the positionof the damper 25 driven by the motor shaft).

The wipers 41 and 36 of potentiometers RI and R22, respectively, areextended to inputs 45 and 46 of comparator circuit means comprising anovel D.C. differen tial amplifier &7 so that the potentialsrepresenting the actual shaft position (wiper 41) and the desired shaftposition (wiper 36) can be compared. When the potentials at amplifierinputs 45 and 46 are equal, the shaft is in the desired position. Whenthe potentials at amplifier inputs 45 and46 are not equal due tomovement of wiper 36 of the command potentiometer by the sensing device26, and the potential difference between the voltages-at inputs 45 and46 is in excess of a predetermined threshold value, the differentialamplifier will provide an enabling signal for drive circuit 48 which isoperable when enabled to effect energization of motor winding 21 andprovide clockwise rotation of the shaft 24, or for drive circuit 49which is operable when enabled to effect energization of motor winding22 and provide counterclockwise rotation of the shaft. The direction ofrotation required to move the shaft toward the desired position isdetermined by the differential amplifier M which senses the polarity ofthe potential difference appearing between inputs 55 and 46 of theamplifier.

The motor shaft 24 rotates so long as the windings 21, 22 are energizedthrough a drive circuit and as the shaft rotates, wiper 41 of the sensepotentiometer R1 is moved, causing the potential at wiper 41 andcorrespondingly at input 45 of the amplifier to increase or decrease.If, for example, the potential at wiper 36 of the command potentiometer(and at input 46 of the differential amplifier) is increased until thepotential difference between amplifier inputs 45 and 46 exceeds thethreshold, an enabling signal will be provided by the amplifier 47 forenabling drive circuit 49 to effect energization of winding 22 with 24volt A.C., while winding 21 is placed in series with capacitor C3 inparallel with winding 22. So long as drive circuit 49 remains enabled,the shaft 24 will rotate counterclockwise moving the damper tocompensate for the change indicated by the sensing device 26, and movingthe wiper 41 of sense potentiometer R1 so that the potential at wiper 41(and at input 45) will be increased as the shaft is rotated, until thepotential difference between amplifier inputs 45 and 46 is less than thethreshold value. At such time, the drive circuit enabling signal will nolonger be provided by the differential amplifier 47 and the energizedmotor windings 22, 21 will be deenergized.

Detent means including resistor R9 connected between winding 21 andamplifier input 45 and resistor R16 connected between winding 22 andamplifier input 46 provide a detent signal for increasing the potentialdifference between amplifier inputs 45 and 46 whenever the windings 21,22 are energized. For example, whenever winding 21 is energized, thevoltage appearing across the other winding 22 is fed back over resistorR16 to input 46 of the amplifier 47. Consequently, the sense signalprovided by the sense controller must overcome both the change inpotential provided by the command controller 30 and the detent signalbefore the potential difference between amplifier inputs and 46 becomesless than the threshold value. A more complete description of theoperation of the motor actuator circuit and more'particularly of thenovel signal comparator means 47 and detent features of the inventionare set forth in the following detailed description.

Detailed Description of Preferred Embodiments Referring to the detailedcircuit schematic diagram of the actuator circuit shown in FIGS. 2 and3, the sense controller means includes a sense potentiometer R1 having atap 50 connected to output terminal 32 of a D.C. voltage supply 34 and aterminal 51 connected over a resistor R2, a diode CR1 and a travel limitadjust potentiometer R4 to output terminal 33 of the voltage supply 34.The command controller means 30 includes a command potentiometer R22having a terminal 52 connected to supply output terminal 32 and aterminal 53 connected to supply output terminal 33'.

As shown in FIG. 3, the D.C. supply 34 includes a voltage rectifiercircuit 55 which provides a halfwave rectified D.C. voltage betweenoutput terminals 32 and 33 of the D.C. voltage supply. The D.C. voltageis derived from a l 10 volt, Hz signal coupled to the motor actuatorcircuit through an input transformer T1 which has a primary winding N1connected to an AC. source. The secondary winding N2 of transformer T1has one end at terminal 58 connected over a unidirectional current pathof the voltage rectifier circuit 55 comprising rectifier diode CR10, afilter network ineluding current limiting resistors R29, R30 and filtercapacitors C5, C6 and diodes CR8, CR9 to output terminal 32. The otherend of winding N2 at terminal 59 is connected over resistor R28 tooutput terminal 33.

The D.C. supply 34 further comprises a voltage regulation circuitincluding transistors Q7 and Q6. The base of transistor O7 is connectedat point 60 to terminal 54 of sense potentiometer R1 and through diodeCR8 to output terminal 32 of the voltage supply, and the emitter oftransistor Q7 is connected to terminal 33. Transistor Q7 acts as a Zenerdiode to provide a stabilized voltage +V across output terminals 32 and33 which voltage appears across the sense controller means 40.Transistor Q6 and associated bias resistors R24-R27 provide furtherregulation for the rectified D.C. voltage and also connect one end ofthe command potentiometer R22 at terminal 33 to output terminal 33 ofthe voltage supply, so that only, say, one-third of the regulated outputvoltage +V appears across the command controller potentiometer R22. Itshould be noted that command controller 30 provides the input commandsignal to the actuator circuit and may be remotely located therefrom. Infact, all that is required is a controller which provides an input D.C.signal across terminals 32 and 70.

Signal Comparator Circuit The amplitudes of the D.C. sense signalprovided at the wiper 41 of feed back potentiometer R1 and the D.C.command signal provided at wiper 36 of the command potentiometer R22 arecompared by the D.C. differential amplifier 47. The differentialamplifier includes an input stage including transistors Q1 and Q4, bothbiased to be normally conducting, and an output stage including outputswitching transistors Q2 and Q5, both normally turned off. The switchingof transistors Q2 and Q5 is controlled by transistors Q1 and Q4 as afunction of the potential present at the differential inputs 45, 46 ofthe amplifier 47.

The base of transistor Q1, which serves as one input 45 to theamplifier, is connected to wiper 41 of the feed back potentiometer R1,and over bias resistor R5 to the D.C. supply 34 at point 60. Thecollector of transistor Q1 is connected through resistor R7 to terminal59.

The base of transistor 04 which serves as a second input 46 to thedifferential amplifier 47 is connected through resistors R20 and R21 towiper 36 of command potentiometer R22 and over bias resistor R19 toterminal 32 of the voltage source 34. The collector of transistor Q4 isconnected through resistor R18 to terminal 59.

The emitters of transistors Q1 and Q4 are connected to the collector oftransistor Q3 which comprises a constant current source 61 for supplyingemitter current to the emitters of transistors Q1 and Q4. The base oftransistor Q3 is connected to terminal 32 of the voltage source, and theemitter of transistor Q3 is connected over resistor R23 to the point 63of the power supply circuit 34 such that forward bias is provided forthe transistor Q3 by diodes CR8 and CR9.

When the input voltages supplied by the sense and command controllersare equal, transistors Q1 and Q4 will share the current from the currentsource 61 equally and the voltages at the collectors of transistors Q1and Q4 will be equal. However, when the input voltages are unequal,current division, and consequently the collector voltages, are unequalso that a differential voltage proportional to the difference in theinput voltage exists between the collectors of transistors Q1 and Q4.This differential voltage is applied to the baseemitter circuits oftransistors Q2 and Q which circuits are reversely connected between thecollectors of transistors Q1 and Q4 such that when the voltage at thecollector of transistors Q] is more positive than the voltage at thecollector of transistor Q4 (by an amount exceeding the minimum turnonvoltage Vbe min. of transistors Q2 and Q5) and the base-emitter circuitof transistor Q5 which is connected between the collector of transistorQ4 and the collector of transistor O1 is forward biased, and thebase-emitter circuit of transistor Q2 which is connected between thecollector of transistor Q2 and the collector of transistor Q4 is reversebiased. In such case, transistor Q5 would be turned on and transistor Q2would remain turned off.

Conversely, if the voltage at the collector of transistors Q1 isnegative relative to the collector of transistor Q4, the base emittercircuit of transistor Q5 would be reverse biased and the base emittercircuit of transistor Q2 would be forward biased.

Motor Drive Circuits Whenever transistor Q2 is forward biased, anenabling signal is provided at the collector of transistor Q2 to enableoutput drive circuit 48 comprised of silicon controlled rectifiers SCR2and SCR3 which, when triggered on, connect the motor winding 21 betweenterminals 58 and 59 for energization by A.C. currents provided overtransformer T1. The switching devices SCR2 and SCR3 are connected ininverse parallel to permit full wave conduction of the energizing A.C.signals.

One end of motor winding 21 is connected directly to terminal 58 and theother end of winding 21 is connected over the anode cathode circuit ofSCR2 to terminal 59. The gate of SCR2 is connected to the collector ofoutput transistor Q2 for receiving an enabling gate signal therefrom fortriggering the SCR2 into conduction. When enabled, SCR2 conducts duringthe positive half of cycles of the power signals permitting A.C. currentto flow from terminal 58 through winding22 and SCR2 to terminal 59. Thesecond switching device SCR3 has its anode-cathode circuit connected ininverse parallel with SCR2 for conducting the A.C. current duringnegative half cycles of the power signal. The gate signal for SCR3, forturning SCR3 on during the negative half cycles of the power signal isprovided by a timing network 65 connected to the gate of SCR3, includingresistors RIO-R12 and capacitor C2 which network is connected inparallel with winding 21 such that capacitor C2 charges during eachpositive half cycle of the power signal, and discharges as SCR2 becomescut off by the negative going power signal, triggering SCR3 intoconduction.

The drive circuit 49 controlled by output switching transistor OS foreffecting energization of winding 22 is similar to drive circuit 48 andincludes inverse connected silicon controlled rectifiers SCRS and SCR6,respectively operable when enabled to connect one end of motor winding22 to terminal 59 during alternate half cycles of the power signal, theother end of the winding being connected to terminal 58. The gate ofSCR6 is connected to the collector of output transistor OS for receivingan enabling gate signal therefrom, and the gate of switching device SCR5is connected to a timing network 66 comprising a capacitor C4 andresistors R13-R15 which provides a gate signal for SCRS. It is pointedout that SCR6 (or SCR2) is enabled to energize winding 22 (or 21)directly and winding 21 (22) over capacitor C3 only at the concurrentpresence of a gate drive signal and a positive anode-cathode voltage.

The motor 23 used is a commercially available 24 VAC single phasepermanent split capacitor motor. A capacitor C3 is connected betweenwindings 21 and 22 of the motor to provide the required phase shift foroperation of the motor on a single phase signal.

One end of winding 21 is connected through detent resistor R9 to thebase of transistor Q1 at the sense input 45 of the differentialamplifier to provide a detent signal whenever winding 22 is energized,which as will be shown hereinafter serves to maintain SCRS and SCR6conducting long enough to assure rotation of the shaft, particularlywhen the potential difference at the amplifier input is small.

Similarly, one end of winding 22 is connected through detent resistorR16 to the base of transistor Q4, the command input 46 of thedifferential amplifier for providing a detent signal whenever winding 21is energized.

Operation of the Motor Actuator Circuit With reference to FIGS. 2 and 3,when the motor shaft 24 is at the desired position the actuator circuitis in a balanced condition and the sense signal provided to the base oftransistor Q1 by the sense controller 40 is approximately equal to thecommand signal provided to the base of transistors Q4 by the commandcontroller 30, the potential difference between the bases of transistorsQ1 and O4 is insufficient to cause either transistor Q2 or Q5 to beforward biased. Under such condition, the motor winding drive circuits48 and 49 are not enabled, and the motor is not energized.

When a change in temperature is sensed by the analog sensing device 26,the wiper 36 of the command controller potentiometer R22 is moved anamount corresponding to the temperature change causing an increase or adecrease in the potential at wiper 36 of potentiometer R22. This changein potential indicates the direction of rotation and the amount ofrotation required for the shaft to position the damper 25 to correspondto the temperature change.

It is pointed out that the voltage at wiper 36 is negative with respectto the voltage at terminal 32. Thus, if the wiper 36 is moved toincrease the potential between wiper 36 and terminal 32, the potentialat wiper 36 will become more negative. Accordingly, the potential at thebase of transistor Q4 will become more negative so that transistor Q4will conduct more emitter current than will transistor Q1, and thecollector voltage of transistor Q4 will be greater than the collectorvoltage of transistor Q1. When the potential difference between thecollectors of transistors Q4 and Q1 exceed Vbe minimum of transistor Q2,transistor Q2 will be forward biased and transistor Q5 will be reversebiased.

When transistor Q2 is forward biased, collector current of transistor Q2will trigger SCR2 of drive circuit 48 into conduction so that winding 21of the motor is connected in a circuit path including the motor winding21 and the secondary winding N2 of the input transformer T1 and theanode to cathode circuit of SCR2 for energization by the A.C. signalsprovided over transformer T1 during the positive half cycles of the A.C.power signals. During the positive half cycles of the power signal,capacitor C2 connected in parallel with resistors R11 and R12, ischarged by current flowing over the series circuit comprising resistorsRIO-R12; connected in parallel with winding 21.

A second energizing path for winding 21 is provided over the anode tocathode circuit of SCR3 and winding N2 during the negative half cyclesof the A.C. power signal, when SCR3 is triggered as capacitor C2discharges over resistors R11, R12.

The silicon controlled rectifiers SCR2 and SCR3 of drive circuit 48 willbe energized during positive and negative half cycles of the powersignal, respectively, as long as output switching transistor Q2 isenabled to provide gate current for SCR2.

When clockwise winding 21 of the motor is energized responsive to theenabling of drive circuit 48, the motor shaft 24 will be moved clockwiseas long as the winding 21 is energized. As the shaft moves clockwisetoward the desired position as indicated by the potential at wiper 36 ofcommand potentiometer R22, wiper 41 of the sense controllerpotentiometer R1 which is mechanically linked to the shaft 24 will alsobe moved in a direction to compensate for the error indicated by thesetting of the command controller potentiometer R22. Thus, while theshaft 24 is rotated clockwise to position the damper 25 as required tocompensate for the change in system conditions, the potential providedat the base of transistor Q1 by the wiper 41 of sense potentiometer R1will approach the potential at the base of transistor Q4 provided by thecommand potentiometer R22. When the potential difference between thecollectors of transistors Q4 and Q1 decreases below Vbe min. oftransistor Q2 the actuator circuit will be in a balanced condition, andoutput transistor Q2 will be cut off such that enabling gate currentwill no longer be supplied to SCR2 of drive circuit 48, and clockwisewinding 21 will be deenergized.

The motor 23, shown schematically in FIG. 2, runs on a single phasevoltage provided from the A.C. source over transformer T1 and extendedto windings 21 or 22 of the motor via switching devices, SCR2, SCR3, andSCRS, SCR6, respectively. The motor capacitor C3 is switched in serieswith either winding 21 or 22 depending upon whether drive circuit 49 or48 is enabled. When the motor is running, both windings 21 and 22 andthe capacitor C3 are energized, but the circuit path which includes thecapacitor C3 and one of the motor windings is nearly at seriesresonance. The voltage across these reactive components is greater thanthe applied voltage and also shifted in phase.

In the present illustration in which the clockwise winding 21 isenergized by the drive circuit 48, the voltage across the other winding22 is approximately twothirds greater than the voltage applied to theclockwise winding 21. This voltage provides the detent signal feed backto the inputs 45, 46 of the differential amplifier 47 to enhance theunbalance caused by an error detection as indicated by the commandcontroller 30. Feedback is provided to both inputs 45, 46, and thedetent signal is the difference in amplitude of the induced voltage fromthat of the 24 volt line.

Thus, in the exemplary illustration, a portion of the induced voltageappearing across the counterclockwise winding 22 is fed back as detentsignal via resistor R16 to the base of transistor Q4, while a lesserportion of the voltage across winding 21 is fed back via resistor R9 tothe base of transistor Q1.

The path for the detent signal provided (when clockwise winding 21 isenergized through drive circuit 48) extends from SCRs 2 and 3, throughcapacitor C3, one end of winding 22 over resistor R16 to the base oftransistor Q4 and through transistor Q3 in parallel with resistors R20and R19 and through voltage rectifier circuit 55 to terminal 58 ofsecondary winding N2 of transformer T1 and thence to the other end ofthe motor winding 22.

The detent signal drives the base of transistor Q4 further negative,relative to the base of transistor Q1 thereby increasing the potentialdifference between the bases of transistors Q1 and Q4 andcorrespondingly the potential difference between the collectors oftransistors Q1 and Q4. It is pointed out that the value of resistor R16is large relative to resistors R20 and R28 such that the detent voltageis on the order of ten to fifteen millivolts which is comparable to thethreshold voltage of the differential amplifier 47 which isapproximately twenty millivolts. The detent signal is A.C. on both addsand subtracts from the potential at the bases of transistors Q1 and Q4.

Accordingly, once SCR2 and SCR 3 of drive circuit 48 are renderedconducting responsive to an enabling signal provided by transistor Q2,the detent signal will assure that the shaft 24 will be rotated enoughto compensate for the change in the setting of the command potentiometerR22 and the actuator circuit will prevent hunting" by the motor for abalanced condition for the actuator circuit.

When the motor winding 21 is energized, the shaft will rotate clockwisetoward the desired position moving the wiper 41 of the sensepotentiometer to decrease the potential at the base of transistor Q1 sothat the potential at the base of transistor Q1 will approach thepotential at the base of transistor Q4 and the potential differencebetween the collectors of transistors Q1 and Q4 will approach zero.

The output transistor Q2 will be cut off when the potential differenceat the input to the amplifier is decreased to a value to cause thepotential difference between collectors of transistor Q1 and Q4 to beless than the value of Vbe min. for transistor Q2, whereby transistor O2is cut off, effecting deenergization of the winding 21 and stoppingrotation of the shaft 24 and further movement of the wiper 41 of thesense potenti ometer R1. The detent signal is not provided when winding21 is deenergized.

The operation of the motor actuator circuit to provide counterclockwiserotation of the shaft is similar to the operation of the actuatorcircuit to provide clockwise shaft rotation. If the opposite commandsignal is provided by the command controller 30, the potential at thebase of transistor Q4 becomes more positive than the potential at thebase of transistor Q1 so that the potential difference between thecollectors of transistors Q1 and Q4 effects reverse biasing oftransistor Q2 and forward biasing of transistor Q5. Accordingly, drivecircuit 49 will be enabled by collector current provided by transistorQ5, rendering SCRS and SCR6 conductive during alternate half cycles ofthe power signal, to connect winging 22 of the motor between terminals58 and 59 for energization by the A.C. power signals. Detent signalsderived from the voltages appearing across the motor windings 21 and 22are fed back over resistors R9 and R16 to the bases of transistors Q1and Q4 in a fashion similar to that described in the foregoing.

At the time when the AC. line is positive with respect to terminal 59,the base-to-emitter voltage of transistor 01 (the potential betweeninput 45 and source output terminal 32) is increased due to the detentvoltage derived across winding 21 and fed back to transistor Q1 to aidin turning SCR6 on for an additional full A.C. cycle.

When winding 22 is energized by the drive circuit 49, the motor shaftwill be rotated clockwise, moving the wiper 41 of feed backpotentiometer R1 to provide a feed back signal for increasing thepotential at the base of transistor Q1 until the potential differencebetween the bases of transistors Q1 and Q4 is decreased below thethreshold value of the differential amplifier 47 at which time the drivecircuit 49 is disabled and motor winding 22 deenergized.

MODIFICATIONS Over-Signal Clamping Diode D1, connected between terminal51 of the sense potentiometer R1 and wiper 36 of the commandpotentiometer R22, provides a means for varying the reference point forpotentiometer R1 relative to the voltage source terminal such thatwhenever the potential at wiper 36 relative to terminal 51 exceeds thevoltage required to forward bias the diode D1, diode D1 conducts so thatthe excess voltage is applied across R1 and adds to the voltage suppliedby the Zener source Q7 between terminals 54 and 51 of the sensepotentiometer.

Travel Limit Adjust The travel limit adjust potentiometer R4 which isconnected in series with the sense controller potentiometer R1 permitsmotor shaft rotation over an angular range of 90 to 270. The maximumvalue of variable resistance element R4 is twice the maximum value ofthe sense resistance element R1. Whenever potentiometer R4 is set atmaximum resistance, the voltage across sense potentiometer R1 isapproximately one-third of the voltage across the Zener device Q7 andthe motor shaft must rotate 270 to provide a sense signal which offsetsthe command and detent signals. If the value of resistance R4 is zero,the full Zener supply, which is three times he maximum voltage availableto the command controller potentiometer R22, appears across R1 and themotor shaft need rotate only one-third of the 270 range to provide anull.

Switching Transistor Drive Circuit FIG. 4 shows an intermediateamplifier stage 67, comprising transistors Q8 and 09, for differentialamplifier 47 which comprises the comparator means. The amplifier circuit67 has terminals A,A' and 13,8 connectable to correspondingly labeledpoints of the differential amplifier circuit which are shown in FIG. 2to be interconnected by links 68 and 69.

When the intermediate stage 67 is connected into the amplifier circuit47, transistor Q8 has its base-emitter circuit connected in place oflink 68 between the collector of transistor Q1 and the emitter oftransistor Q5, and transistor Q9 has its base-emitter circuit connectedin place of link 69 between the collector of transistor 04 and theemitter of transistor Q2. The collectors of transistors Q8 and Q9 areconnected to the positive terminal 32 of the voltage source.

When a positive potential difference exists between the collectors oftransistors Q1 and Q4, transistors Q8 and Q5 will be forward biased andtransistors Q9 and Q2 reverse biased. When such potential difference isnegative, transistors Q9 and Q2 will be forward biased and transistorsQ8 and Q5 will be reversed biased.

With the intermediate stage 67, the differential amplifier 47 provides ahigher value output control signal since the input stage furnishes onlythe base drive current for the output transistors Q2 and Q5. Thecollector current for the output transistors Q2 and Q5 is provided bythe intermediate stage 67.

External Controllers Several motor actuator circuits can be slaved fromone command input means, such as command controller 30. In addition, theDC. supply voltage provided between terminal 32 and terminal 59 can beused to energize external controllers requiring an unregulated supplyvoltage.

It is pointed out that an. externally generated command signal providedby a command input means can be applied to the motor actuator circuitacross terminals and 32 by connecting the output of a variety ofsuitable controllers, such as a differential transformer or operationalamplifier output, in place of the output of the controller potentiometerR22. In fact, all that is required is that a DC. input command signal beprovided by suitable command input means across terminals 70 and 32 ofthe activate control circuits.

The differential transformer will be controlled by a sensing device,such as device 26, to provide an output representing the desiredpositions for the shaft.

Input Signal Characterizer In certain installations, the devicepositioned by the motor shaft may have non-linear operatingcharacteristics. For example, if the damper 25 were a valve, a linearchange in the command input signal provided by the sensing device 26would effect a linear change in the valve set point. However, a linearchange in the valve set point would not provide a correspondingly linearchange in the flow rate through the valve.

Therefore, as shown in FIG. 1, an adjustable characterizing resistor R0is shown connected in series with an enabling switch Sc across the sensecontroller potentiometer R1 permitting linearization of the valveoperating characteristic.

Thus, when switch Sc is closed to connect the characterizing resistanceRc in parallel with the sense potentiometer R1, the value of resistanceRe can be adjusted to cause the control circuit to provide nonlinearchanges in shaft position in response to a linearly changing commandinput signal, thereby compensating for the non-linear characteristic ofthe valve positioned by the motor and providing a linear change in flowrate for a linear change in the command input signal provided by thesensing device 26.

I claim:

1. In a motor control circuit for effecting selective energization offirst and second windings of a motor to rotate the motor shaft to adesired position, command means for providing a command input signalrepresenting the desired position for said shaft, sense controller meansfor providing a sense signal representing the actual position of saidshaft, comparator means having first and second inputs for receivingsaid command and sense signals, respectively, said comparator meansbeing responsive to a change in said command signal relative to saidsense signal to effect the energization of aid first and second windingsselectively to cause rotation of said shaft in a direction toward saiddesired position, and detent means for extending first and second detentsignals derived from said first and second windings to the first andsecond inputs, respectively, of said comparator means whenever saidwindings are energized to increase the difference between said commandand sense signals supplied to the first and second inputs of saidcomparator means.

2. A motor control circuit as set forth in claim 1 wherein said sensecontroller means includes signal characterizing means operable whenenabled to modify said sense signal whereby a linear change in saidcommand input signal effects non-linear change in the shaft position.

3. A motor control circuit as set forth in claim 2 wherein said sensecontroller means includes first potentiometer means and said signalcharacterizing means includes adjustable resistor means and means forconnecting said adjustable resistor means in parallel with said firstpotentiometer means.

4. In a motor control circuit for effecting selective energization offirst and second windings of a motor to rotate the motor shaft to adesired position, command input means for providing a command inputsignal representing the desired position for said shaft, sensecontroller means for providing a sense signal representing the actualposition of said shaft, comparator means responsive to a difference inthe amplitudes of said signals to effect energization of said windingsto rotate said shaft in a direction toward the desired position, anddetent means including first circuit means connected between one of saidmotor windings and said command input means to provide a detent signalto said command input means whenever said windings are energized andsecond circuit means connected between the other of said motor windingsand said sense controller means to provide a detent signal to said sensecontroller means whenever said windings are energized, to therebyincrease the potential difference between said command and sense signalssupplied to said comparator means.

5. A motor control circuit as set forth in claim 3 wherein saidcomparator means includes first output means enabled whenever saidcommand signal increases a predetermined amount relative to said sensesignal to energize said windings to effect rotation of the shaft in onedirection, and second output means enabled whenever said command signaldecreases a predetermined amount relative to said sense signal toenergize said windings to effect rotation of said shaft in the oppositedirection.

6. A motor control circuit as set forth in claim 3 including over signalclamping means connected between said sense controller means and saidcommand input means.

7. In a motor control circuit for effecting selective energization offirst and second windings of a motor to rotate the motor shaft to adesired position, command controller means for providing a commandsignal representing the desired position for the shaft, sense controllermeans for providing a sense signal representing the actual position ofthe shaft, comparator means including switching means operable wheneversaid command and sense signals differ by a given amount to provide anoutput signal, the polarity of the output signal being different as thecommand signal increases or decreases relative to the sense signal,first drive means responsive to an output signal of one polarity toenergize said windings to effect rotation of the shaft in one directionand second drive means responsive to an output signal of a secondpolarity to energize said windings to effect rotation of the shaft inthe opposite direction and detent means including first circuit meansconnected to provide a first detent signal derived from said firstwinding to said command controller means whenever said windings areenergized, and second circuit means connected to provide a second detentsignal derived from said second winding to said sense controller meanswhenever said windings are energized, said first and second detentsignals being coupled to said command and sense controller meansrespectively, to increase the difference between said command and sensesignals, said sense controller means being controlled by said motorshaft such that as said shaft rotates towards said desired position, theamplitude of the sense signal approaches the amplitude of the commandsignal, to thereby decrease the difference between said com mand andsense signals below said given amount and effect deenergization of saidwindings.

8. A motor control circuit as set forth in claim 6 wherein said commandcontroller means includes first potentiometer means connected acrossoutput terminals of a voltage source for providing said command signalat a wiper of said first potentiometer means and wherein said sensecontroller means includes second potentiometer means connected acrosssaid source output terminals for providing said sense signal at a wiperof said second potentiometer means.

9. A motor control circuit as set forth in claim 8 wherein said firstcircuit means comprise resistance means connected between said firstwinding and said first potentiometer wiper to couple said detent signalto said first potentiometer wiper whenever said second winding isenergized, and said second circuit means comprise second resistancemeans connected between said second winding and said secondpotentiometer wiper to couple said detent signal to said potentiometerwiper whenever said first winding is energized.

10. A motor control circuit as set forth in claim 8 including diodemeans connected between said resistance means of said first circuitmeans and a terminal of said second potentiometer means for assuringthat a sense signal greater in amplitude than the command signal canalways be obtained.

11. A motor control circuit as set forth in claim 6 wherein said commandcontroller means includes first potentiometer means connected across apair of output terminals of a voltage source which provide a firstpotential across said command controller means, and said sensecontroller means includes second potentiometer means and limitpotentiometer means serially connected across a further pair of outputterminals of said source which provide a second potential across saidsense controller means, said limit potentiometer means being adjustableto vary the potential across said second potentiometer means betweensaid first potential and said second potential.

12. In a motor control circuit for effecting selective energization offirst and second windings of a motor to rotate the motor shaft to adesired position, command controller means for providing a commandsignal of an amplitude representing the desired position for the shaft,sense controller means for providing a sense signal of an amplituderepresenting the actual position of the shaft, comparator meansincluding switching means operable whenever the amplitudes of thecommand and sense signals differ by a given amount to provide an outputsignal, the polarity of the output signal being different as the commandsignal increases or decreases relative to the sense signal, first drivemeans enabled responsive to an output signal of one polarity to effectenergization of said windings to effect rotation of the shaft in onedirection and second drive means enabled responsive to an output signalof a second polarity to effect energization of said windings to effectrotation of the shaft in the opposite direction and detent meansincluding first circuit means connected between one of said windings andsaid command controller means to provide a first detent signal derivedfrom said one winding whenever said windings are energized and secondcircuit means connected between the other of said windings and saidsense controller means to provide a second detent signal derived fromsaid other winding whenever said windings are energized, said first andsecond detent signals being coupled to said command and sense controllermeans, respectively to increase the difference between said command andsense signals, said sense controller means being controlled by saidmotor shaft such that as said shaft rotates towards said desiredposition, the amplitude of the sense signal approaches the amplitude ofthe command signal decreasing the difference between the amplitude ofsaid command and sense signals below said given amount to thereby effectdeenergization of said windings.

13. In a motor control circuit for effecting selective energization offirst and second windings of a motor to rotate the motor shaft to adesired position, command controller means for providing a commandsignal representing the desired position for the shaft, sense controllermeans for providing a sense signal representing the actual position ofthe shaft, comparator means including a first input transistor having aninput electrode connected to said sense controller means to receive saidsense signal and a first control electrode, a second input transistorhaving an input electrode connected to said command controller means toreceive said command signal and a second control electrode, a firstswitching transistor having a first input electrode connected to saidfirst control electrode and a second input electrode connected to saidsecond control electrode, and a second switching transistor having afirst input electrode connected to said second control electrode and asecond input electrode connected to said first control electrode, andmeans forward biasing said first switching transistor and reversebiasing said second switching transistor to provide an output signal ofa first polarity whenever the amplitude of the command signal exceedsthe amplitude of the sense signal by a predetermined amount of reversebiasing said first switching transistor and forward biasing said secondswitching transistor to provide an output signal of the oppositepolarity whenever the amplitude of the sense signal exceeds theamplitude of the command signal by said predetermined amount, firstdrive means responsive to one of said output signals to energize saidwindings to effect rotation of said shaft in one direction, second drivemeans responsive to the other one of said output signals to energizesaid windings to effect rotation of said shaft in the oppositedirection, and detent means including first circuit means connected toprovide a first detent signal derived from one of said windings to saidcommand controller means and second circuit means connected to provide asecond detent signal derived from the other of said windings to saidsense controller means, said first and second detent signals beingcoupled to said command and sense controller means, respectively tothereby increase the difference in amplitude between said command andsense signals.

14. A motor control circuit for effecting selective energization offirst and second windings of a motor to rotate the shaft to a desiredposition, said motor control circuit comprising command input mans forproviding a first signal representing the desired position for theshaft, sense controller means for providing a second signal representingthe actual position of the shaft, signal comparator means includinginput means responsive to a difference in said first and second signalsto provide a potential difference of a first or a second polaritybetween first and second output which indicates an increase or adecrease in the first signal relative to said second signal, and outputmeans with first and second switching means having control circuitsreversely connected between said first and second outputs such that whenthe potential difference between said outputs is one polarity, saidfirst switching means is enabled and said second switching means isdisabled to provide a first output signal, and when the potentialdifference between said outputs is of a second polarity, said secondswitching means is enabled and said first switching means is disabled toprovide a second output signal, first drive means responsive to saidfirst output signal to effect energization of one of said motorwindings, and a second drive means responsive to said second outputsignal to effect energization of the other of said motor windings, anddetent means including first circuit means connected between one of saidmotor windings and said sense controller means for coupling a detentsignal derived from said one windings to said sense controller meanswhenever said windings are energized and second circuit means connectedbetween the other one of said motor windings and said command inputmeans for coupling a further detent signal derived from said other motorwinding to said command input means whenever said windings are energizedto thereby increase the difference between said command and sensesignals.

15. In a motor control circuit for effecting selective energization offirst and second windings of a motor to rotate the motor shaft to adesired position, signal comparator means, command means for supplying acommand signal representing the desired position for said shaft to aninput of said comparator means, sense controller means for supplying asense signal representing the actual position of said shaft to a furtherinput of said comparator means, said comparator means being responsiveto a change in said command signal relative to said sense signal toselectively energize said windings to cause rotation of said shaft in adirection toward said desired position, and signal characterizing meansconnected to said sense controller means and operable when enabled tomodify the sense signal provided by said sense controller means wherebya linear change in said command signal relative to said sense signaleffects a non-linear change in the position of said motor shaft. 3 9 =1

1. In a motor control circuit for effecting selective energization offirst and second windings of a motor to rotate the motor shaft to adesired position, command means for providing a command input signalrepresenting the desired position for said shaft, sense controller meansfor providing a sense signal representing the actual position of saidshaft, comparator means having first and second inputs for receivingsaid command and sense signals, respectively, said comparator meansbeing responsive to a change in said command signal relative to saidsense signal to effect the energization of aid first and second windingsselectively to cause rotation of said shaft in a direction toward saiddesired position, and detent means for extending first and second detentsignals derived from said first and second windings to the first andsecond inputs, respectively, of said comparator means whenever saidwindings are energized to increase the difference between said commandand sense signals supplied to the first and second inputs of saidcomparator means.
 2. A motor control circuit as set forth in claim 1wherein said sense controller means includes signal characterizing meansoperable when enabled to modify said sense signal whereby a linearchange in said command input signal effects non-linear change in theshaft position.
 3. A motor control circuit as set forth in claim 2wherein said sense controller means includes first potentIometer meansand said signal characterizing means includes adjustable resistor meansand means for connecting said adjustable resistor means in parallel withsaid first potentiometer means.
 4. In a motor control circuit foreffecting selective energization of first and second windings of a motorto rotate the motor shaft to a desired position, command input means forproviding a command input signal representing the desired position forsaid shaft, sense controller means for providing a sense signalrepresenting the actual position of said shaft, comparator meansresponsive to a difference in the amplitudes of said signals to effectenergization of said windings to rotate said shaft in a direction towardthe desired position, and detent means including first circuit meansconnected between one of said motor windings and said command inputmeans to provide a detent signal to said command input means wheneversaid windings are energized and second circuit means connected betweenthe other of said motor windings and said sense controller means toprovide a detent signal to said sense controller means whenever saidwindings are energized, to thereby increase the potential differencebetween said command and sense signals supplied to said comparatormeans.
 5. A motor control circuit as set forth in claim 3 wherein saidcomparator means includes first output means enabled whenever saidcommand signal increases a predetermined amount relative to said sensesignal to energize said windings to effect rotation of the shaft in onedirection, and second output means enabled whenever said command signaldecreases a predetermined amount relative to said sense signal toenergize said windings to effect rotation of said shaft in the oppositedirection.
 6. A motor control circuit as set forth in claim 3 includingover-signal clamping means connected between said sense controller meansand said command input means.
 7. In a motor control circuit foreffecting selective energization of first and second windings of a motorto rotate the motor shaft to a desired position, command controllermeans for providing a command signal representing the desired positionfor the shaft, sense controller means for providing a sense signalrepresenting the actual position of the shaft, comparator meansincluding switching means operable whenever said command and sensesignals differ by a given amount to provide an output signal, thepolarity of the output signal being different as the command signalincreases or decreases relative to the sense signal, first drive meansresponsive to an output signal of one polarity to energize said windingsto effect rotation of the shaft in one direction and second drive meansresponsive to an output signal of a second polarity to energize saidwindings to effect rotation of the shaft in the opposite direction , anddetent means including first circuit means connected to provide a firstdetent signal derived from said first winding to said command controllermeans whenever said windings are energized, and second circuit meansconnected to provide a second detent signal derived from said secondwinding to said sense controller means whenever said windings areenergized, said first and second detent signals being coupled to saidcommand and sense controller means , respectively, to increase thedifference between said command and sense signals, said sense controllermeans being controlled by said motor shaft such that as said shaftrotates towards said desired position, the amplitude of the sense signalapproaches the amplitude of the command signal, to thereby decrease thedifference between said command and sense signals below said givenamount and effect deenergization of said windings.
 8. A motor controlcircuit as set forth in claim 6 wherein said command controller meansincludes first potentiometer means connected across output terminals ofa voltage source for providing said command signal at a wiper of saidfirst potentiometer means and wherein said senSe controller meansincludes second potentiometer means connected across said source outputterminals for providing said sense signal at a wiper of said secondpotentiometer means.
 9. A motor control circuit as set forth in claim 8wherein said first circuit means comprise resistance means connectedbetween said first winding and said first potentiometer wiper to couplesaid detent signal to said first potentiometer wiper whenever saidsecond winding is energized, and said second circuit means comprisesecond resistance means connected between said second winding and saidsecond potentiometer wiper to couple said detent signal to saidpotentiometer wiper whenever said first winding is energized.
 10. Amotor control circuit as set forth in claim 8 including diode meansconnected between said resistance means of said first circuit means anda terminal of said second potentiometer means for assuring that a sensesignal greater in amplitude than the command signal can always beobtained.
 11. A motor control circuit as set forth in claim 6 whereinsaid command controller means includes first potentiometer meansconnected across a pair of output terminals of a voltage source whichprovide a first potential across said command controller means, and saidsense controller means includes second potentiometer means and limitpotentiometer means serially connected across a further pair of outputterminals of said source which provide a second potential across saidsense controller means, said limit potentiometer means being adjustableto vary the potential across said second potentiometer means betweensaid first potential and said second potential.
 12. In a motor controlcircuit for effecting selective energization of first and secondwindings of a motor to rotate the motor shaft to a desired position,command controller means for providing a command signal of an amplituderepresenting the desired position for the shaft, sense controller meansfor providing a sense signal of an amplitude representing the actualposition of the shaft, comparator means including switching meansoperable whenever the amplitudes of the command and sense signals differby a given amount to provide an output signal, the polarity of theoutput signal being different as the command signal increases ordecreases relative to the sense signal, first drive means enabledresponsive to an output signal of one polarity to effect energization ofsaid windings to effect rotation of the shaft in one direction andsecond drive means enabled responsive to an output signal of a secondpolarity to effect energization of said windings to effect rotation ofthe shaft in the opposite direction and detent means including firstcircuit means connected between one of said windings and said commandcontroller means to provide a first detent signal derived from said onewinding whenever said windings are energized and second circuit meansconnected between the other of said windings and said sense controllermeans to provide a second detent signal derived from said other windingwhenever said windings are energized, said first and second detentsignals being coupled to said command and sense controller means,respectively to increase the difference between said command and sensesignals, said sense controller means being controlled by said motorshaft such that as said shaft rotates towards said desired position, theamplitude of the sense signal approaches the amplitude of the commandsignal decreasing the difference between the amplitude of said commandand sense signals below said given amount to thereby effectdeenergization of said windings.
 13. In a motor control circuit foreffecting selective energization of first and second windings of a motorto rotate the motor shaft to a desired position, command controllermeans for providing a command signal representing the desired positionfor the shaft, sense controller means for providing a sense signalrepresenting the actual position of the shaft, comparatoR meansincluding a first input transistor having an input electrode connectedto said sense controller means to receive said sense signal and a firstcontrol electrode, a second input transistor having an input electrodeconnected to said command controller means to receive said commandsignal and a second control electrode, a first switching transistorhaving a first input electrode connected to said first control electrodeand a second input electrode connected to said second control electrode,and a second switching transistor having a first input electrodeconnected to said second control electrode and a second input electrodeconnected to said first control electrode, and means forward biasingsaid first switching transistor and reverse biasing said secondswitching transistor to provide an output signal of a first polaritywhenever the amplitude of the command signal exceeds the amplitude ofthe sense signal by a predetermined amount of reverse biasing said firstswitching transistor and forward biasing said second switchingtransistor to provide an output signal of the opposite polarity wheneverthe amplitude of the sense signal exceeds the amplitude of the commandsignal by said predetermined amount, first drive means responsive to oneof said output signals to energize said windings to effect rotation ofsaid shaft in one direction, second drive means responsive to the otherone of said output signals to energize said windings to effect rotationof said shaft in the opposite direction, and detent means includingfirst circuit means connected to provide a first detent signal derivedfrom one of said windings to said command controller means and secondcircuit means connected to provide a second detent signal derived fromthe other of said windings to said sense controller means, said firstand second detent signals being coupled to said command and sensecontroller means, respectively to thereby increase the difference inamplitude between said command and sense signals.
 14. A motor controlcircuit for effecting selective energization of first and secondwindings of a motor to rotate the shaft to a desired position, saidmotor control circuit comprising command input mans for providing afirst signal representing the desired position for the shaft, sensecontroller means for providing a second signal representing the actualposition of the shaft, signal comparator means including input meansresponsive to a difference in said first and second signals to provide apotential difference of a first or a second polarity between first andsecond output which indicates an increase or a decrease in the firstsignal relative to said second signal, and output means with first andsecond switching means having control circuits reversely connectedbetween said first and second outputs such that when the potentialdifference between said outputs is one polarity, said first switchingmeans is enabled and said second switching means is disabled to providea first output signal, and when the potential difference between saidoutputs is of a second polarity, said second switching means is enabledand said first switching means is disabled to provide a second outputsignal, first drive means responsive to said first output signal toeffect energization of one of said motor windings, and a second drivemeans responsive to said second output signal to effect energization ofthe other of said motor windings, and detent means including firstcircuit means connected between one of said motor windings and saidsense controller means for coupling a detent signal derived from saidone windings to said sense controller means whenever said windings areenergized and second circuit means connected between the other one ofsaid motor windings and said command input means for coupling a furtherdetent signal derived from said other motor winding to said commandinput means whenever said windings are energized to thereby increase thedifference between said command and sense signals.
 15. In A motorcontrol circuit for effecting selective energization of first and secondwindings of a motor to rotate the motor shaft to a desired position,signal comparator means, command means for supplying a command signalrepresenting the desired position for said shaft to an input of saidcomparator means, sense controller means for supplying a sense signalrepresenting the actual position of said shaft to a further input ofsaid comparator means, said comparator means being responsive to achange in said command signal relative to said sense signal toselectively energize said windings to cause rotation of said shaft in adirection toward said desired position, and signal characterizing meansconnected to said sense controller means and operable when enabled tomodify the sense signal provided by said sense controller means wherebya linear change in said command signal relative to said sense signaleffects a non-linear change in the position of said motor shaft.